Fix crash for FEniCS-OpenFOAM perpendicular flap example with subcycling (#554)

* Don't work on vectors but use `assign` function

Author: NiklasVin

changelog-entries/554.md

@@ -0,0 +1 @@
+- Use `assign` function instead of directly accessing vectors in perpendicular-flap/solid-fenics. Fixes problem with checkpointing.

perpendicular-flap/solid-fenics/solid.py

@@ -158,16 +158,14 @@ def update_v(a, u_old, v_old, a_old, ufl=True):
 def update_fields(u, u_old, v_old, a_old):
     """Update all fields at the end of a timestep."""
 
-    u_vec, u0_vec = u.vector(), u_old.vector()
-    v0_vec, a0_vec = v_old.vector(), a_old.vector()
-
     # call update functions
-    a_vec = update_a(u_vec, u0_vec, v0_vec, a0_vec, ufl=False)
-    v_vec = update_v(a_vec, u0_vec, v0_vec, a0_vec, ufl=False)
+    a_new = update_a(u, u_old, v_old, a_old)
+    v_new = update_v(u, u_old, v_old, a_old)
 
-    # assign u->u_old
-    v_old.vector()[:], a_old.vector()[:] = v_vec, a_vec
-    u_old.vector()[:] = u.vector()
+    # update values
+    a_old.assign(project(a_new, V))
+    v_old.assign(project(v_new, V))
+    u_old.assign(u)
 
 
 def avg(x_old, x_new, alpha):
@@ -238,7 +236,6 @@ def avg(x_old, x_new, alpha):
         n = n_cp
     else:
         update_fields(u_np1, u_n, v_n, a_n)
-        u_n.assign(u_np1)
         t += float(dt)
         n += 1